Geophysical electromagnetic (EM) techniques may be effective in the determination of the electrical conductivity of soils, rocks and other conductive material at depths from the surface up to about three kilometers. Conductivity distribution at such depths is of great interest to those involved in mapping base metal and uranium deposits, aquifers and other geological formations.
Geophysical EM methods involve measurement of time-varying magnetic fields near the earth's surface produced by a primary magnetic field and modeling of the ground conductivity distributions. These magnetic fields are generated either by a periodic current applied to a transmitter, or by naturally occurring electromagnetic fields originating mainly from lightning in the earth's, atmosphere. EM fields can have a characteristic ground penetration depth proportional to the inverse of the square-root of both ground conductivity and frequency.
In known methods, the magnetic field signal is measured using either a receiver coil system (which can measure up to three orthogonal components of the magnetic field time-derivative dB/dt), or a magnetometer (which measures the magnetic field B). The received analog signal is then amplified, filtered, and digitized by a high-resolution high-speed analog-to-digital converter (ADC), and the data may be stored along with the positioning information obtained from a Global Positioning System (GPS). Data post-processing involves electrical and physical, modeling of the ground to generate the geophysical conductivity contour maps.
Existing geophysical surveying methods typically require high signal-to-noise ratio (SNR), high conductivity discrimination, and high spatial resolution both laterally and in depth.
Existing EM systems encompass both ground-based and airborne measurements. Airborne measurements are collected through the use of airplanes and helicopters. Airborne methods are, useful for large area surveys and may be used for exploration of conductive ore bodies buried in resistive bedrock, geological mapping, hydrogeology, and environmental monitoring. Known airborne electromagnetic (AEM) systems function so that the data is acquired while the airplane or helicopter flies at nearly constant speed (e.g. up to 75 m/s, or 30 m/s, respectively) along nearly-parallel equally-spaced lines (e.g. 50 m to 200 m) at close to constant height above ground (e.g. about 120 m or 30 m, respectively). Measurements are taken at regular intervals, typically in the range, 1 m up to 100 m.
An additional feature of known EM measurements is that they can be achieved either in the frequency domain or time domain. In FDEM measurements, the transmitter coil continuously transmits an electromagnetic signal at fixed multiple frequencies, while the receiver coil measures the signal continuously over time. The measured quantities are signal amplitude and phase as a function of frequency, or equivalently, the in-phase and in quadrature amplitudes as a function of frequency. In these measurements, the signal sensitivity is reduced with increasing conductivity, thus reducing the conductivity contrast mapping.
In the course of collecting time TDEM measurements by known methods, a pulse of current is applied to the transmitter coil during an on-period and switched off during the off-period, typically at a repetition rate equal to an odd multiple of half of the local, power line frequency (e.g. typically 50 Hz or 60 Hz). The signal is measured at the receiver as a function of time. The signal amplitude decay during the off-period, combined with modeling of the conductivity and geometry of geological bodies in the ground, yields the conductivity contour maps.
In known TDEM systems, during the current-on-period, weak conductors produce weak dB/dt signals at the receiver coil while good conductors produce large in-phase signals, although quite small compared to the unwanted primary EM field generated by the transmitter coil system. During the current-off-period, weak conductors produce a large dB/dt signal at the receiver coil from a rapidly decaying EM field while good conductors, produce small signals from a slowly decaying EM field. Measurements are: typically made during the off-period, and while measurement of dB/dt is useful to map weak conductors, the measurement of the magnetic field, referred to as the B-field, can increase the accuracy of information provided for good conductors.
In known methods the magnetic field B can be obtained either by direct measurement using a magnetometer or by time-integrating the signal dB/dt measured with a receiver coil. When the magnetic field B is to be obtained by integration, the dB/dt response over the full waveform has to be measured including during the on-period, in order to determine the integration constant that provides a zero DC component over the entire period (see Smith, R. S. and Annan 4.P., “Using an induction coil sensor to indirectly measure the B-field response in the bandwidth of the transient electromagnet method”, Geophysics, 65, p. 1489-1494).
An example of a TDEM HTEM system that measures the magnetic filed time derivative dB/dt can be seen for example in U.S. Pat. No. 7,157,914, the contents of which are, incorporated herein by reference.
A TDEM system that can be efficiently operated while effectively measuring the B-Filed is desirable.